Drink about a gallon a day for the next 2-3 days. Lie quietly in a darkened room. Call your doctor and request a narcotic medication, and perhaps something for nausea (my headaches always morphed into nausea as well.)

If it becomes the worst headache of your life, call the on-call doctor or go to ER.

Sorry you're going through this. Next time have them run the IVIG slower.